Such a system may be a system of the GSM type, which provides the possibility for a subscriber possessing such a mobile station to connect to another system than the one it is initially connected to by subscription. In GSM terms this is called roaming. On this subject could notably be consulted the publication: "The GSM System for Mobile Communication" by Michel Mouly and Marie-Bernadette Pautet, published by the authors whose address is 4 rue Elisee Reclus. F-91120 France.
The problem with which the subscriber is confronted is that of determining, even without the roaming procedure, another system that is suitable for him and of which he could take a subscription. Although the subscriber already has an initial communication system, he could be tempted to be connected to another system that would suit him better, for example, he could choose a system that offers a less costly subscription. However, he is not to forget that this less expensive system is to have a sufficiently large radio coverage area to satisfy his needs for communication when he moves.